1. Field of the Invention
The invention relates to a process for preparing fine-particle rubber powders from rubber-latex emulsions by precipitation from an aqueous mixture having a filler. Further, the invention relates to the resulting powders.
2. Discussion of the Background
A wide variety of publications has dealt with the purpose and reasons for the use of rubber powders, and also with processes for the preparation of rubber powders.
The interest in pulverulent rubber/filler masterbatches can be explained as a necessary result of the processing technology used in the rubber industry, where rubber mixtures are prepared with high costs for energy, time and personnel. A main reason for this is that the rubber raw material is in bale form and its processing requires incorporation and dispersion of large amounts of active fillers (industrial carbon blacks, silicas, etc.) into the rubber phase.
This mechanical kneading process generally takes place industrially in large internal mixers or on roll mills, generally in two or more stages. The mixture is placed in intermediate storage between the individual mixing stages, and finally, after addition of the crosslinking chemicals the ready-to-use raw mixture is further processed in extruder systems or on calenders to give semifinished products (e.g. tire treads).
Only a completely new process technology can overcome this complicated method of rubber processing.
Rubber powder technology has long been regarded as the most suitable candidate. [Delphi Report xe2x80x9cKxc3xcnftige Herstellverfahren in der Gummiindustriexe2x80x9d [Production processes for the future in the rubber industry] Rubber Journal, Vol. 154, No. 11, 20-34 (1942)]. This technology combines the need for incorporating a filler at an early stage of the process with presentation of the material specifically in the form of a free-flowing rubber powder suitable for use in continuous mixing processes which have been state of the art for many years in the plastics industry.
The rubber powder is prepared as in the prior art by precipitation from a mixture of a filler suspension (e.g. carbon black or silica) and a rubber-latex emulsion, by lowering the pH with the aid of appropriate Brxc3x6nsted or Lewis acids (DE-C 37 23 213, DE-C 37 23 214 and DE-C 28 22 148).
All of these patents regard it as essential to add water-soluble salts of a metal of Groups IIa, IIb, IIIa or VIII of the Periodic Table of the Elements, in amounts of from 0.5 to 6.5 phr. In more recent patents, DE 198 15 453.4 and DE 198 16 972.8, the addition of 10 phr of water-soluble salts of a metal of groups IIa, IIb, IIIa or VIII to the filler suspension is described.
This Group classification corresponds to the IUPAC recommendation (see Periodisches System der Elemente [Periodic Table of the Elements], Verlag Chemie, Weinheim, 1985). Typical examples for water-soluble salts of the above metals are magnesium chloride, zinc sulfate, aluminum chloride, aluminum sulfate, iron chloride, iron sulfate, cobalt nitrate and nickel sulfate. Aluminum salts are preferably used and more preferably aluminum sulfate.
The presence of an alkali metal silicate in amounts of up to 5 phr is also indispensable according to the prior art.
However, all of these non rubber-specific substances that are added in large amounts during preparation of the rubber powder have to be regarded as unnecessary and in some cases even as injurious constituents of the product. These non rubber-specific substances are partially incorporated into the product and a large part remains in the precipitation water. Thus, they are potential environmental pollutants. Complicated treatment of the wastewater is needed in some instances. The aim is therefore to prepare rubber/filler masterbatches by using, as far as possible, only substances which are desirable in rubber technology and/or improve vulcanizate performance, or which at least have no adverse effect (Cabot patent, PCT/US98/20279).
The object of the present invention is to develop a precipitation and preparation process for a fine-particle, pulverulent, filled rubber which is free from the undesirable ancillary constituents mentioned above and at the same time fulfills the other requirements placed upon the rubber powder, such as control of tack, and excellent vulcanizate properties.
This and other objects have been achieved according to the invention, the first embodiment of which includes a process for preparing a fine-particle, pulverulent, filled rubber, comprising:
precipitating said fine-particle, pulverulent, filled rubber from an aqueous mixture of a filler suspension and a rubber-latex emulsion in the absence of a water-soluble salt of a metal of Groups IIa, IIb, IIIa or VIII of the Periodic Table of the Elements, and in the absence of an alkali metal silicate.
The invention describes a novel process for preparing pulverulent, free-flowing rubber/filler masterbatches. The process is characterized by the following procedure.
At the start of preparation of the novel rubber powders, the entirety of the filler is divided into three portions: core black, splitting black 1 and splitting black 2. The core black, which makes up from about 30 to 60% of the entirety of the carbon black in the final product, is dispersed in water. The total amount of water depends on the nature of the filler and on its degree of disaggregation. The amount of water-insoluble constituents in the filler is generally from 1 to 10%, preferably from 4 to 7%. The amount of water-insoluble constituents includes all values and subvalues therebetween, especially including 2, 3, 4, 5, 6, 7, 8 and 9%. The filler content of the suspension acts in the absence of the abovementioned bivalent salts as a regulator for the particle size distribution that is desired in the final product. Generally, higher solids content in the suspension gives a coarser product, and lower solids content yields a finer product.
Two different precipitation procedures can then follow the preparation of these filler suspensions.
1. Precipitation using a Precipitation Tube (Semicontinuous Precipitation)
For this, a Bronsted acid, preferably sulfuric acid, is used to adjust the filler suspension described above which is composed of the core black at an early stage to the final pH value for the precipitation of from 2.5 to 6, preferably from 3.8 to 5.5. The suspension is then run through a tubular reactor together with the rubber-latex emulsion. This initiates coagulation of the latex and formation of the rubber powder grains, and the grains formed are collected in a tank. The pH value of the precipitation includes all values and subvalues therebetween, especially including 3, 3.5, 4, 4.5, 5 and 5.5.
At the same time, an aqueous suspension with a pH of from 7 to 9.5, preferably from 7.5 to 8.5, made from the second portion of the carbon black (splitting black 1) composed of from 30 to 60% of the entirety of the carbon black is fed continuously to the rubber powder grains formed in the collector. Again in parallel with this, continuous addition of 10% sulfuric acid is used to maintain the pH of the rubber powder suspension at the desired final pH for the precipitation, i.e. from 2.5 to 6, preferably from 3.8 to 5.5, and the precipitation is completed under these conditions. The grain size of the rubber powder is controlled via the continuous feed of the splitting black 1.
Once the precipitation has ended, the third portion of the carbon black (splitting black 2), consisting of from 5 to 15% of the entirety of the carbon black, is added, in the form of a filler suspension, to the aqueous rubber powder suspension. Optionally, sulfuric acid is again used to adjust the final pH value within the above-mentioned range.
2. Premix Precipitation
First, a filler suspension is prepared by dispersing a portion of the filler present in the final product preferably from 50 to 98%, in water. The portion of the filler includes all values and subvalues therebetween, especially including 55, 60, 65, 70, 75, 80, 85, 90 and 95%. The solids content of the suspension is from 1 to 10%, preferably from 4 to 7%, depending on the nature of the filler and on its degree of disaggregation. The solids content includes all values and subvalues therebetween, especially including 2, 3, 4, 5, 6, 7, 8 and 9%. The pH of the suspension here is from 7 to 9.5, preferably from 7.5 to 9. The pH includes all values and subvalues therebetween, especially including 7.5, 8, 8.5 and 9.
The resultant filler suspension, together with the entirety of the rubber-latex emulsion, is charged to a precipitation tank and vigorously stirred. A 10% H2SO4 solution is then fed to carry out the precipitation, while the final pH of the precipitation suspension is brought within a range from 2.5 to 6, preferably from 3.8 to 5.5. Splitting black 1 (from 2 to 50% of the entirety of the carbon black), likewise suspended in water, is then added. Optionally, H2SO4 is again used to adjust the pH.
The rubber powder is preferably precipitated at a temperature of from 10 to 60xc2x0 C., and more preferably at an ambient temperature of from 20 to 35xc2x0 C.
Preferred rubber types are emulsion SBR, acrylonitrile rubber, aqueous-emulsion polymerized butadiene rubber and natural rubber which are used individually or in a mixture.
In the case of natural rubber, a particularly preferred form is that known as xe2x80x9cfield latexxe2x80x9d with Mooney viscosities of from 50 to 120 Mooney units, and preferably of from 55 to 75 Mooney units.
The solids content of the rubber emulsions is preferably of from 15 to 65%, and more preferably from 20 to 30%. The solids content includes all values and subvalues therebetween, especially including 20, 25, 30, 35, 40, 45, 50, 55 and 60%.
The fillers comprise the carbon blacks known from rubber processing. These include furnace blacks, gas blacks and flame blacks with an iodine adsorption value of from 5 to 1000 m2/g, a C TAB value of from 15 to 600 m2/g, a DBP adsorption of from 30 to 400 ml/100 g and a 24 M4 DBP value of from 50 to 370 ml/100 g, in amounts of from 5 to 250 parts by weight, preferably from 20 to 150 parts by weight, per 100 parts by weight of rubber, and preferably per 40 to 100 parts by weight of rubber.
In a preferred embodiment, the products of the invention comprise carbon blacks of application DE 198 40 663 having improved dynamic properties compared to standard carbon blacks.
In this case the total amount of filler is from 20 to 250 parts of filler per 100 parts of rubber. The rubber powders of the invention can comprise, besides the abovementioned carbon black fillers, precipitated silicas and naturally occurring fillers, organosilanes and, if desired, known processing or vulcanization auxiliaries, such as zinc oxide, zinc stearate, stearic acid, polyalcohols, polyamines, plasticizers, aging inhibitors to protect from the action of heat, light or oxygen and ozone, reinforcing resins, flame retardants, e.g. Al(OH)3 and Mg(OH)2, pigments, various crosslinking chemicals and, if desired, sulfur, in the concentrations usual for vulcanization.
The rubber powders present in water and obtained by the above procedure are then substantially dewatered by mechanical means. Preferred assemblies are centrifuges or vacuum-belt filters.
Finally, heat-drying takes place, preferably in a fluidized-bed dryer, to reduce the moisture to a value of xe2x89xa63%, preferably xe2x89xa61%.
In another preferred embodiment, it has been proven to be advantageous for the finished, dried rubber powder to be powdered, i.e. dry-coated, with suitable products used in the rubber industry for a further reduction in tack. Suitable products include stearic acid, silicas and in particular zinc oxide, in amounts of from 0.1 to 3 phr, and preferably of from 0.25 to 1 phr. The application of the coating layer onto the rubber powder grains takes place in suitable powder mixers, in which the use of shear is avoided in order not to damage the grains.
The applied coating layer particularly allows storage over prolonged periods with high compaction of the product (e.g. in a silo, without blocking).
The rubber powders of the invention are used for preparing vulcanizable rubber mixtures. All of the constituents needed to prepare the mixture may be present here in the rubber powder. However, the constituents preferably comprise rubber of the grades listed above and fillers, but they may also be mixed in a conventional manner with other rubbers and with fillers if this is a requirement for the desired rubber mixture.
The fine-particle rubber powders of the invention are easy to process, give excellent dispersions in vulcanizing mixtures, and give vulcanizates with improved properties.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.